The present invention relates to unequal path interferometers, and more particularly to a plurality of associated unequal path interferometers. Interferometers constructed in accordance with the concepts of this invention are adapted, among other possible uses, for use in detecting, and determining the wavelength of coherent radiation, as from a laser, in a quantum of radiation including incoherent radiation. It is particularly adapted for discriminating coherent radiation from incoherent radiation, and determining wavelength, from a single pulse or from a continuous wave of the radiation. In addition, it is adapted to determine the relative angular position of the source of the radiation. A preferred form of the apparatus according to the invention includes a Fabry-Perot etalon having a plurality of regions of different thicknesses.
A Fabry-Perot etalon interferometer consists normally of two plane, parallel partially reflecting surfaces formed on a solid glass spacer so that one portion of incident radiation is transmitted directly through while other portions, being reflected between the partially reflecting surfaces before emerging, are transmitted over a longer path.
As described in U.S. Pat. No. 3,824,018 to R. Crane, and assigned to the same assignee as the present application, a Fabry-Perot etalon is adapted to discriminate coherent radiation by making the optical thickness of the glass spacer sufficiently less than the absolute coherence length of the incident radiation (ie. the absolute of the coherence length of the coherent and incoherent incident radiation) so that the absolute coherent length will be substantially less than the difference between the lengths of the paths of the directly and indirectly transmitted radiation (ie. the optical path difference or OPD). Then, by changing the path length, by means of tilting the etalon, the intensity of the incoherent radiation transmitted will remain constant, but the coherent radiation modulates. The intensity of radiation transmitted through the etalon is a function of the OPD and of the wavelength of the radiation. The OPD is a function of the index of refraction of the spacer, of the thickness of the spacer and the angle of refraction of the radiation passing through the interior of the spacer. As the etalon is tilted in a scanning mode at a predetermined rate to vary the optical path lengths, and the OPD, the intensity of the transmitted radiation, suitably detected by a photodetector at the back of the etalon, varies in a manner such that the frequency of the radiation as picked up by the detector decreases and reaches a minimum as the etalon swings through a position at which the angle of incidence of the impinging radiation, and hence the angle of refraction is zero. By this means the etalon is utilized to detect the relative position of the source of the coherent radiation. Additionally, this etalon may be used to determine the wavelength of the coherent radiation at a preselected scan rate with reference to coherent radiation of known wavelength and comparing the detected frequency-wavelength pattern with the pattern similarly produced by the coherent radiation whose wavelength is known.
The aforementioned patent also disclosed a two stepped etalon to, in effect, provide two etalon regions of different thicknesses. As disclosed, the thicknesses differ by a quarter wavelength of the wavelength of the coherent radiation to be detected so that the optical path differences of the two etalon regions differ by a half wavelength. A detector is provided for each of the two etalon portions. The output signals from the two detectors, which are made to be of opposite polarity are added, so that the constant output signal components produced by the incoherent radiation cancel out and the coherent radiation signals are accentuated. Otherwise, the detected output is analyzed in the same manner as described with reference to the single etalon embodiment.
While the device of the aforementioned patent operated with reasonable success in certain installations having a continuous wave source, my contribution to the art is a new etalon interferometer, which is an improvement over such prior devices, which is capable of processing a single pulse to distinguish the characteristics thereof, and which has other advantages, as will become apparent as the description proceeds.